The ionosphere is a layer of charged particles (or "ions") located about 60-90 kilometers above the Earth's surface. It is the layer which reflects short wave radio waves and makes it possible for a transmitter in New York to send a signal to Tokyo. Short waves may be good enough to send The Voice of America broadcasts, but they are subject to noise and static. Short wave signals lose much of their power as they travel and can even be blocked by the weather and sunspot activity.